py(deps): Add pytest-xdist

[tony]

Changes

Add pytest-xdist dev package

See also:

• https://pypi.org/project/pytest-xdist/
• https://github.com/pytest-dev/pytest-xdist

See also

• https://github.com/tmux-python/libtmux/pull/522

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.19%. Comparing base (296276d) to head (6f5c073).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #930 +/- ## ======================================= Coverage 70.19% 70.19% ======================================= Files 26 26 Lines 1936 1936 Branches 404 404 ======================================= Hits 1359 1359 Misses 454 454 Partials 123 123 ```

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